In a typical cellular radio system, also referred to as a wireless communication system, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “NodeB” or “B node” or “eNodeB” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies which is denoted Evolved Universal Terrestrial Radio Access Network E-UTRAN also referred to the 3GPP work item on the Long Term Evolution (LTE).
In mobile communications, when a wireless terminal is in connected mode and travels through a radio access network, the wireless terminal is served by different cells in the course of the travel. That is, as a wireless terminal travels through the radio access network, the wireless terminal is typically handed over, e.g., from one cell to another cell, through a handover procedure. This involves large number of “mobility parameters”, which should be properly tuned and optimized to ensure robust mobility performance.
Downlink Measurements for Mobility User equipment unit mobility primarily relies on downlink measurements. Downlink measurements used in various systems are described below:
In WCDMA the following three downlink radio related measurements are specified primarily for mobility reasons:
(1) Common Pilot CHannel Received Signal Code Power (CPICH RSCP);
(2) CPICH Ec/No, this is basically the signal-to-noise ratio used for representing the “cell quality” for handover evaluation;
(3) UTRA Carrier Received Signal Strength Indicator (RSSI).
The first two of the above measurements are performed by the user equipment unit on cell level basis on the Common Pilot CHannel (CPICH). The UTRA carrier RSSI is measured over the entire carrier. The above CPICH measurements are the main quantities used for the mobility decisions. In addition, in WCDMA several timing related measurements are also specified for connected mode mobility procedure. They are used to adjust the user equipment timing when performing handover in connected mode.
In E-UTRAN the following three downlink quality measurements are specified primarily for mobility reasons:
(1) Reference Symbol Received Power (RSRP);
(2) Reference Symbol Received Quality (RSRQ);
(3) E-UTRA Carrier RSSI.
The first two of the above measurements are performed by the user equipment on cell level basis on reference symbols. As in case of WCDMA, the E-UTRA carrier RSSI is measured over the entire carrier. The two RS based measurements are indeed also the main quantities, which are likely to be used for the mobility decisions.
Handover Evaluation Processes
Handover is controlled by the network through explicit UE specific commands and by performance specification. In connected mode the UE regularly performs measurements on neighbor cells. The network configures the UE to report events associated with mobility when certain conditions are fulfilled, e.g., when neighbour cell becomes stronger than the serving cell by some margin, i.e. hysteresis, over certain time, i.e., time hysteresis. The UE reports events and/or measurement report to the network, which in turn makes an appropriate decision, e.g. sends a handover command to the UE. Typical connected mode mobility parameters are a layer-3 or higher layer filtering coefficient for additional time domain filtering, time to trigger (time hysteresis), and hysteresis (for signal).
It is very important the handover process is successful, since if the handover failures it may lead to a dropped session or interrupted service. In LTE, the UE performs downlink measurement periodically based on reference symbols (RS), also referred to as reference signals and Layer 1 and Layer 3 filtering are then applied. Based on the processed measurement, a Layer 3 event evaluation takes place. When the event triggering condition is fulfilled, UE sends the measurement report to source eNB which processes the report and makes handover decision. The handover will be performed if it is approved at source eNB. The first phase of performing handover in LTE is the Handover Preparation which includes the signaling exchange between source eNB and target eNB via an X2 interface and admission control of UE in the target eNB. After UE receiving Handover Command message from source eNB, the second phase Handover Execution begins. During this phase, UE performs Random Access and synchronization to the target eNB. If it succeeds, target eNB will assign the uplink allocation by sending a grant to UE and UE responds with the HO confirm message which indicates the completion of the HO procedure at the radio access network part.
A handover may fail due to the delay of sending the involved signaling messages including the UE measurement report, handover command and handover confirm. In addition, handover may fail when performing random access to the target cell fails.
Common reasons for high handover failure rate may e.g. be coverage limitations or high system load combined with high UE speed. Many of the handover failures occur when a UE transmits measurement report to source eNB. These failures are due to the accentuated resource limitation in the UpLink (UL), i.e. the radio link from the UE to the eNB, or due to coverage limitations. In those “difficult mobility scenarios”, the UL signaling message is segmented into several segments and the headers are added which increase the transmission delay and result in the handover failure.